1. Field of the Invention
This invention is concerned with a method for recovery of petroleum from viscous petroleum-containing formations including tar sand deposits, and more particularly is concerned with an improved multiple-component solvent flooding technique especially useful in viscous petroleum-containing formations including tar sand deposits.
2. Description of the Prior Art
There are many subterranean petroleum-containing formations in various parts of the world from which petroleum cannot be recovered by conventional means because the petroleum is too viscous to flow or be pumped. The most extreme example of viscous petroleum-containing formations are the so-called tar sand or bituminous sand deposits. The largest and most famous such formation is the Athabasca Tar Sand Deposit in the northeastern part of the Province of Alberta, Canada, which contains over 700 billion barrels of petroleum. Other extensive deposits are known to exist in the western United States and Venezuela, and smaller deposits exist in Europe and Asia.
Tar sands are defined as sand saturated with a highly viscous crude petroleum material not recoverable in its natural state through a well by ordinary production methods. The petroleum constituent of tar sand deposits are highly bituminous in character. The sand present in tar sand deposits is generally fine quartz sand coated with a layer of water, with the bituminous petroleum material occupying most of the void space around the water wetted sand grains. The balance of the void space is filed with connate water, and some deposits contain small volumes of gas such as air or methane. The sand grains are packed to a void volume of about 35 percent, which corresponds to 83 percent by weight sand. The balance of the material is bitumen and water, and the sum of bitumen and water is fairly consistantly 17 percent by weight, with the bitumen portion thereof varying from about 2 percent to about 16 percent. One of the characteristics of tar sand deposits which differs considerably from conventional petroleum-containing formations is the absence of a consolidated material matrix within the formation. The sand grains are in contact although uncemented and the bitumen occupies most of the void space. The API gravity of the bitumen ranges from about 6 to about 8, and the specific gravity at 60.degree.F is from about 1.006 to about 1.027. Generally around 50 percent of the bitumen is distillable without cracking. The bituminous petroleum from tar sand deposits is so viscous that some on-site refining of the produced petroleum must be undertaken if the material is to be pumped in an unheated pipeline.
The methods for recovering bituminous petroleum from tar sand deposits include strip mining and in situ separation processes. Most of the recovery to date has been by means of strip mining, although this is economically feasible only when the ratio of overburden thickness to tar sand deposit thickness is around 1 or less. Vast quantities of petroleum are known to exist in the form of tar sand deposits which are not within a range which is economically suitable for strip mining, and so there is a serious need for some form of in situ process wherein the bitumen or bituminous petroleum is separated from the sand by some means and recovered therefrom through a well or other production means drilled into the tar sand deposit.
In situ processes proposed in the literature may be categorized as thermal techniques, including fire flooding and steam flooding, and emulsification drive processes. To be successful, an in situ separation process must accomplish two functions: the viscosity of the crude oil must be reduced and sufficient driving energy to induce movement of the crude oil must be supplied. Emulsification processes frequently also employ steam, plus a basic material such as sodium hydroxide which induces formation of an oil-in-water emulsion having a viscosity substantially lower than the viscosity of the formation petroleum. Thermal processes are restricted to formations having sufficient overburden thickness to permit injection of high pressure fluids. Many tar sand deposits exist in which the overburden thickness is too thin for thermal flooding and too thick for strip mining.
One possible process for recovering bitumen from tar sand deposits which does not require the presence of sufficient overburden thickness to contain high pressures is solvent flooding. Solvent flooding involves injection of a solvent into the tar sand deposit, which solvent dilutes and reduces the viscosity of the bituminous petroleum to render it mobile and recoverable by means of a well as is normally employed in conventional oil recovery operations. Although many solvents including aromatic hydrocarbons such benzene, toluene and xylene, as well as carbon tetrachloride or carbon disulfide, readily dissolve bituminous petroleum these materials are expensive and since huge quantities are required, solvent flooding has not been considered to be economically feasible. Paraffinic hydrocarbons such as propane, butane, pentane, etc. are more readily available and less expensive than those solvents described above, but it has always been uniformly assumed by persons skilled in the art that paraffinic hydrocarbon solvents could not be used in bituminous petroleum because of the danger of precipitating asphaltenes. Indeed, the asphaltic constituents of crude oil are frequently defined as pentane-insoluble materials.
It can be seen from the foregoing that there is a substantial need for a method for recovering viscous petroleum such as bitumen or bituminous petroleum from a tar sand formation by use of readily available inexpensive solvents in a relatively low pressure procedure that can be used in intermediate deep deposits as well as in deep deposits.